Mechanistic Studies of the Formation of Functionalized Cyclopropanes through Photoisomerization of Tropolone Diels-Alder Adducts

Our search for novel chemotheraputic agents against parasitic diseases has led to a series of bridged hydrazines (Figure 1, 1a-e). 1-5 Some of these compounds display promising in vitro activities against Trypanosoma cruzi and Leishmania donovani. To expand the structural diversity of our libraries, and based on results obtained for similar β,γ-unsaturated ketone chromophores, 7,8 we studied the photoisomerization these alkyltropolone derivatives. As detailed herein, irradiation of these compounds leads to functionalized cyclopropanes in good yields. Mechanistic studies revealed that the process is stereospecific, proceeds through the formation of allyl radical and ketene intermediates, and involves participation of the solvent.